1. Field of the Invention
The present invention relates generally to inspection of electrical generators using electromagnetic detectors and more particularly to electromagnetic detectors that vary the frequency of the excitation current to provide information on the depth of detected flaws.
2. Related Art
The stator core of a typical large generator, for example, a 500 megawatt generator, weighs 200 tons, is approximately 6 meters long, 2.6 meters in diameter and has a bore of 1.3 meters. The stator is built from a stack of approximately 200,000 individual steel sheets or laminations, each approximately 0.3 mm thick and coated, for example, with a varnish to insulate it electrically from adjacent laminations and the windings that are inserted in peripheral slots that extend circumferentially around the laminations. The core is held together on its outside by steel building bars. On its inside, it carries a winding made from electrically insulated copper bars embedded in slots between the rows of inward facing teeth around the bore.
Within the bore of the stator lies the rotor, which is spun by mechanical power of the turbines to induce electrical power in the stator winding. The rotor itself carries a winding, which is energized by direct current provided by an exciter. As the magnetic flux produced by this winding rotates, it intersects the stator winding and generates the alternating current power, which is the generator""s required output. The function of the steel laminations is to ensure that the stator core presents a path of low magnetic impedance to the spinning rotor flux.
It is vital, however, to prevent unwanted currents being generated in the steel of the core (as opposed to the wanted currents in the stator winding). The result would be serious overheating in the core. This is why the laminations are each coated with a thin layer of electrical insulation. The insulation on a group of laminations may, however, become damaged near the bore surface during assembly, operation or maintenance. If this happens, a conducting circuit may be completed, since in many stators, the laminations are also in electrical contact with each other at their outer edges where they are supported by the building bars. The rotating flux will then induce currents around these circuits which can lead to troublesome overheating or hot spots in the damaged area. Hot spots usually occur on or near the stator teeth.
If allowed to persist, the hot spots can damage or possibly cause failure of the electrical insulation around the conductors of the stator winding, necessitating replacement of the conductor. There have been instances where hot spots have grown to such an extent that the core itself has had to be rebuilt.
Primitive forms of hot spot detection at the surface involved exciting the core to operating flux density by means of a temporary High Power Ring Flux Loop (HPRFL). This technique uses a heavy gauge cable loop installed such that it extends through the bore of the stator, then around the outside of the frame, and then through the bore again. Three to ten turns are normally required. The loop is energized with a high voltage and observers are positioned in the bore to manually examine the surface of the stator core. If the area to be examined is limited, the HPRFL method can be used to excite the core after the suspected area is treated with paraffin or paints that change color when heated.
A thermographic inspection technique is an alternative to the hands-on observation of stator damage. This technique also employs the HPRFL to excite the core to operational flux density levels. The entire surface area of the core structure can then be scanned with a television-style camera that is sensitive to infrared radiation. The entire examination is done from the outside end of the core looking into the bore, but it is often desirable to de-energize the HPRFL for a short time to enter the bore and pinpoint sources of heat.
More recently, electromagnetic detectors such as the Electromagnetic Core Imperfection Detector (EL CID) described in U.S. Pat. No. 5,321,362 have been employed for this purpose. This technique employs an excitation loop of #10 AWG 300-volt wire (usually 6 turns) installed in the bore of the stator core, often suspended along the centerline and around the frame in a manner similar to that of the HPRFL technique. The loop is then connected to a source of constant frequency amplitude-adjustable ac voltage (a 240-volt Variac) and energized. A separate single turn search coil determines when the proper level of excitation is obtained. The flux level is approximately 4% of the operating flux density. At this low density, technicians can safely enter the bore with a pickup device that detects axial currents in the laminations or the pickup device can be inserted remotely with small robots such as that described in U.S. Pat. No. 5,557,216, assigned to the assignee of the instant application. The pickup is moved over the entire bore surface in a series of overlapping patterns while the output is observed on a meter and/or plotted on an X-Y recorder or computer. Any areas of elevated axial current in the laminations along the surface or some distance below the surface will be indicated as peaks on the output device. The need for corrective action can be determined objectively by analyzing the peaks. This technique is more fully described in a publication, Sutton, J., July 1980. Electrical Review, Vol.207, No. 1, xe2x80x9cEL CID: An Easier Way to Test Stator Coresxe2x80x9d, 33-37. The outputs of the pickup coil can be further processed and analyzed by a computer, which can compare the information to known reference values to assist in characterizing the flaw that was identified. The results provide information on the location of the flaw but not its radial depth.
An improved electromagnetic detector is desired that will provide additional information to further characterize a flaw.
Accordingly, it is an object of this invention to provide an improved electromagnetic detector that can further characterize a flaw in the insulation of the laminations of a stator core.
It is an additional object of this invention to provide such an improved electromagnetic detector that can further characterize the depth of any flaw detected.
It is a further object of this invention to provide an electromagnetic generator stator insulation flaw detector that will speed up the detection process.
These and other objects are achieved by an electromagnetic detector that employs an excitation coil like that employed with the EL CID technique, which is connected to a variable frequency ac source and generates approximately 4% of the normal operating flux density of a turbine stator. A pickup coil is moved axially along the core at a given circumferential elevation and the output of the pickup coil is interpreted to identify the locations of flaws. The output of the pickup coil is indexed against the axial position of the coil to identify the location of any flaw, which is indicated. The output thus obtained is recorded as the pickup coil is moved from one axial end of the generator stator core to the other, defining a single pass or scan. Multiple measurements are recorded from the pickup coil at different frequencies. Preferably, one at the line frequency, e.g., 60 or 50 hertz, and one at 2000 hertz. If a flaw is detected at a given location, the number of measurements at that location is increased, each respectively at a different frequency.
Preferably, a scan is made at a single frequency and then a second scan, optionally in the reverse direction, is made at a second frequency. When the series of scans is completed across a given circumferential area over the axial length of the stator core, the pickup coil is circumferentially displaced, desirably to the next adjoining area to be scanned that has not as yet been monitored and the process is repeated until the entire surface area of the interior of the stator core has been monitored.
In the preferred embodiment, a plurality of pickup coils respectively monitor adjacent regions simultaneously. The adjacent pickup coils are circumferentially aligned as they traverse the stator core axially. The output of the plurality pickup coils is preferably multiplexed to a computer for storage, analysis and display. Desirably, the plurality of outputs are displayed together. In another embodiment, more than one pickup coil is employed at a given circumferential location with each coil having a different, but overlapping monitored area. The coils can employ multiple windings tuned to the different frequencies to optimize the coil output.